The present invention relates to a hydrogen absorbing alloy powder and a process for producing the same.
It is a conventional practice that a hydrogen absorbing alloy powder is produced through various steps such as melting, casting, thermal treatment, pulverizing and classification, and the hydrogen absorbing alloy powder is used after being subjected to an activating treatment.
However, the conventional hydrogen absorbing alloy powder suffers from a problem that because the powder is subjected to the activating treatment requiring a lot of time and much amount of heat, the production cost is correspondingly high.
In addition, in general, the hydrogen absorbing alloy powder is used in a fine powdery state having a particle size equal to or smaller than 100 xcexcm, and hence, such powder is liable to be oxidized in the atmospheric air. To prevent the oxidation, the hydrogen absorbing alloy powder must be maintained and handled in an inert atmosphere, but this brings about the degradation of the handling operability.
Accordingly, it is an object of the present invention to provide a process for producing a hydrogen absorbing alloy powder by utilizing peculiar mechanical alloying, wherein a hydrogen absorbing alloy powder, which has an excellent PCT characteristic without being subjected to the conventional activating treatment and can be handled in the atmospheric air without hindrance, can be produced.
To achieve the above object, according to the present invention, there is provided a process for producing a hydrogen absorbing alloy powder, comprising the steps of throwing a starting powder into a ball mill, in which an AE powder comprising at least one alloy element AE selected from the group consisting of Ti, V, Mn, Fe, Ni, Cu and Al, and an Mg powder are weighed so that an alloy composition comprises an amount of AE in a range of 0.1% by weightxe2x89xa6AExe2x89xa620% by weight and the balance of Mg, and subjecting the starting powder to a mechanical alloying in a hydrogen atmosphere to provide a hydrogen absorbing alloy powder comprising an Mg alloy powder.
When the hydrogen absorbing alloy powder is hydrogenated, the most time and amount of heat are required in a first hydrogenating course after the production of the hydrogen absorbing alloy powder. For this reason, the conventional activating treatment is a consideration.
In the present invention, the mechanical alloying is carried out in the hydrogen atmosphere and hence, the hydrogen absorbing alloy powder after being produced contains a large amount of a metal hydride, namely, the hydrogen absorbing alloy powder is one produced as if via the first hydrogenating course in the conventional activating treatment. Therefore, if the hydrogen absorbing alloy powder is subjected to a dehydrogenating thermal treatment in vacuum or in a hydrogen atmosphere, hydrogen is desorbed from the metal hydride, and the hydrogen absorbing alloy powder, which has not been hydrogenated, is also activated simultaneously by the migration of the desorbed hydrogen between the grains. In this manner, the sufficiently activated hydrogen absorbing alloy powder can be produced. This hydrogen absorbing alloy powder has an excellent PCT characteristic.
Moreover, the hydrogen absorbing alloy powder containing the large amount of the metal hydride is stable, as compared with a hydrogen absorbing alloy powder containing no metal hydride. Therefore, even if the hydrogen absorbing alloy powder is handled in the atmospheric air, an adverse influence to the PCT characteristic due to the oxidation and the like is inhibited largely. However, if the content of the alloy element AE is lower than 0.1% by weight, the amount of very small grains produced, present in Mg crystal grains or grain boundaries, is insufficient and hence, an excellent hydrogen-absorbing/desorbing characteristic (PCT characteristic) cannot be obtained. On the other hand, if AE greater than 20% by weight, Vf (the volume fraction) of the matrix is reduced and hence, a hydrogen storage capacity equal to or higher than 6% by weight cannot be obtained.
According to the present invention, there is provided a process for producing a hydrogen absorbing alloy powder, comprising the steps of throwing a starting powder into a container of a ball mill, and charging hydrogen into the container to conduct a mechanical alloying, hydrogen being charged again into the container at an intermediate stage of the mechanical alloying.
With such process, the hydrogen absorbing alloy powder can be hydrogenated sufficiently.
It is another object of the present invention to provide a hydrogen absorbing alloy powder having a high hydrogenating speed and a high dehydrogenating speed without being subjected to an activating treatment, and having an improved thermodynamic characteristic.
To achieve the above object, according to the present invention, there is provided a hydrogen absorbing alloy powder, which comprises an amount of nickel (Ni) in a range of 2.1% by weightxe2x89xa6Nixe2x89xa647.2% by weight, an amount of AE in a range of 0.1% by weightxe2x89xa6AExe2x89xa616.3% by weight and the balance of Mg, the AE being at least one alloy element selected from the group consisting of Ti, V, Mn, Fe, Zr and Cu, and a plurality of very small grains having an average grain size d equal to or smaller than 20 nm being dispersed in each of Mg crystal grains constituting a matrix and in each of grain boundaries. The term xe2x80x9cgrain size dxe2x80x9d means a length of the longest portions of the very small grains in a photomicrographic structure diagram (or a photomicrograph showing a metallographic structure). This applies to the Mg crystal grains and the like.
The very small grains are produced by adding the alloy element AE to the Mgxe2x80x94Ni based alloy component and subjecting them to the mechanical alloying. The very small grains exist stably, and the coalescence of the very small grains is not observed in hydrogen-absorbing/releasing courses.
Such very small grains have an effect of promoting the adsorption of hydrogen molecules to surfaces of the Mg crystal grains and the dissociation of the adsorbed hydrogen molecules to hydrogen atoms in the hydrogen-absorbing course. Due to a difference between distances between atom faces generated between Mg atoms and the very small grains within each of the Mg crystal grains, an elastic strain field is produced in an interface area of the very small grains, and is an energetically high and highly active area. A plurality of such highly active areas exist in the Mg crystal grains and hence, an inactive Mg phase is activated, whereby the diffusion of hydrogen atoms into the Mg crystal grains is promoted. In this manner, the hydrogenating speed is increased.
On the other hand, in the hydrogen desorption course, the diffusion of the hydrogen atoms into the surfaces of the Mg crystal grains is promoted due to the presence of the highly active areas, and the very small grains promote the production of hydrogen molecules by bonding of the hydrogen atoms and the desorption of the hydrogen molecules from the surfaces of the Mg crystal grains. In this manner, the dehydrogenating speed is increased.
The metallographic structure with the very small grains having the average grain size d equal to or smaller than 20 nm being dispersed within each of the Mg crystal grains is a nano-sized composite structure and hence, the structural stability of the hydride MgH2 is inhibited. Namely, in this alloy, the thermodynamic characteristic for MgH2 is improved, and a drop in temperature of hydrogen dissociation thereof is achieved.
The content of Ni is set as described above in order to ensure that the Mgxe2x80x94Ni based alloy has a function as a hydrogen absorbing alloy. If the content of Ni is smaller than 2.1% by weight, such function is declined. On the other hand, if the content of Ni larger than 47.2% by weight, the entire matrix is formed mainly of Mg2Ni and for this reason, the nano-sized composite structure cannot be produced.
If the content of the alloy element AE is smaller than 0.1% by weight, the amount of very small grains produced is insufficient. On the other hand, if AE greater than 16.3% by weight, the coalescence of the very small grains is brought about and for this reason, the hydrogen absorption/desorption characteristic of the powder is degraded. The content of the alloy element AE is preferably equal to or smaller than 5.5% by weight.
It is a yet further object of the present invention to provide a hydrogen absorbing alloy powder having a high hydrogenating speed and a high dehydrogenating speed without being subjected to an activating treatment, and having improved thermodynamic characteristic and durability.
To achieve the above object, according to the present invention, there is provided a hydrogen absorbing alloy powder comprising an amount of AE in a range of 0.1% by weightxe2x89xa6AExe2x89xa620% by weight and the balance of Mg, the AE being at least one alloy element selected from the group consisting of Ti, V, Mn and Fe, an average grain size D of Mg crystal grains constituting a matrix being equal to or smaller than 500 nm (Dxe2x89xa6500 nm), and a plurality of very small grains having an average grain size d equal to or smaller than 20 nm being dispersed in each of the Mg crystal grains and grain boundaries. The term xe2x80x9cthe grain size of Mg crystal grainsxe2x80x9d means a length of the longest portions of the Mg crystal grains in a photomicrographic structure diagram (or a photomicrograph showing a metallographic structure), and the average value of the lengths is the average grain size D of the Mg crystal grains. The term xe2x80x9cgrain size d of the very small grainsxe2x80x9d likewise means a length of the longest portions of the very small grains.
The metallographic structure, in which the plurality of very small grains having the average grain size d equal to or smaller than 20 nm are dispersed in each of the Mg crystal grains having the average grain size D equal to or smaller than 500 nm and in each of grain boundaries, is a nano-sized composite structure. Such structure is formed by adding the particular amount of the AE powder comprising the alloy element AE to the Mg powder, and subjecting the resulting powder mixture to the mechanical alloying in a hydrogen atmosphere and then to a dehydrogenating thermal treatment in vacuum or in a hydrogen atmosphere. The very small grains exist stably and cannot be coalesced in hydrogen-absorbing and desorbing courses at about 300xc2x0 C. and hence, the coalescence of the Mg crystal grains is also inhibited. Namely, the nano-sized composite structure lasts over a long period.
In such nano-sized composite structure, the very small grains have an effect of promoting the adsorption of hydrogen molecules to surfaces of the Mg crystal grains and the dissociation of the adsorbed hydrogen molecules to hydrogen atoms in the hydrogen-absorbing course. Due to a difference between distances between atom faces generated between Mg atoms and the very small grains within each of the Mg crystal grains, an elastic strain field is produced in an interface area of the very small grains, and is an energetically high and highly active area. A plurality of such highly active areas exist in the Mg crystal grains and hence, an inactive Mg phase is activated, whereby the diffusion of hydrogen atoms into the Mg crystal grains is promoted. In this manner, the hydrogenating speed is increased. In addition, because the content of Mg is larger than 80% by weight, the hydrogen storage capacity is increased to approximately 6% by weight or more.
On the other hand, in the hydrogen desorption course, the diffusion of hydrogen atoms to the surfaces of the Mg crystal grains is promoted due to the presence of the highly active areas, and the very small grains promote the production of hydrogen molecules by bonding of hydrogen atoms and the desorption of the hydrogen molecules from the surface of the Mg crystal grains. In this manner, the dehydrogenating speed is increased.
The hydrogenating speed and the dehydrogenating speed which are excellent as described above are maintained over a long period with the lasting of the nano-sized composite structure and hence, the hydrogen absorbing alloy has an excellent durability.
In the nano-sized composite structure, the structural stability of the hydride MgH2 is inhibited. Namely, the thermodynamic characteristic for the MgH2 is improved, and a reduction in temperature of hydrogen-dissociation thereof is achieved.
However, if the content of the alloy element AE is smaller than 0.1% by weight, the amount of very small grains produced is insufficient. On the other hand, if AE greater than 20% by weight, the volume fraction (Vf) of the matrix is decreased and hence, a high hydrogen storage capacity as described above cannot be obtained.
It is a yet further object of the present invention to provide a producing process of the above-described type, wherein a hydrogen absorbing alloy powder having a high hydrogenating speed and a high hydrogen storage capacity and moreover having such a characteristic that a dehydrogenating speed is high, can be produced without being subjected to an activating treatment, and the thermodynamic characteristic and the durability of the hydrogen absorbing alloy powder can be improved.
To achieve the above object, according to the present invention, there is provided a process for producing a hydrogen absorbing alloy powder, comprising the steps of weighing an AE powder comprising at least alloy element AE selected from the group consisting of Ti, V, Mn, Fe and Ni, and an Mg powder to provide an alloy composition comprising an amount of AE in a range of 0.1% by weightxe2x89xa6AExe2x89xa620% by weight and the balance of Mg, and throwing the AE powder and the Mg powder into a ball mill, where they are subjected to a mechanical alloying in a hydrogen atmosphere and then to a dehydrogenating thermal treatment either in vacuum or in a hydrogen atmosphere, thereby providing a hydrogen absorbing alloy powder which includes a plurality of Mg crystal grains constituting a matrix and having an average grain size D equal to or smaller than 500 nm, and a plurality of very small grains having an average grain size d equal to or smaller than 20 nm and dispersed in each of the Mg crystal grains and in each of grain boundaries. The term xe2x80x9cgrains size of Mg crystal grainsxe2x80x9d means a length of the longest portions of the Mg crystal grains in a photomicrographic structure diagram (or a photomicrograph showing a metallographic structure), and the average value of the lengths is the average grain size D of the Mg crystal grains. The term xe2x80x9cgrain size d of the very small grainsxe2x80x9d likewise means a length of the longest portions of the very small grains.
Thus, the hydrogen absorbing alloy powder having the metallographic structure, namely, having the nano-sized composite structure, with a plurality of the very small grains having the average grain size d equal to or smaller than 20 nm being dispersed in each of the Mg crystal grains having the average grain size D equal to or smaller than 500 nm and in each of grain boundaries can be easily produced by conducting the mechanical alloying in the hydrogen atmosphere and then the dehydrogenating thermal treatment either in vacuum or in the hydrogen atmosphere, as described above.